Multilevel Inverter with Natural Balancing of DC Sources for PV System Applications

This paper presents a photovoltaic system as a standalone system. The proposed system consists of photovoltaic (PV) module, battery energy storage system (BESS), high frequency transformer, and multilevel inverter (MLI). The MLI consists of two H-bridge single phase inverter per arm (phase) with unequal dc voltages which needs 6 isolated dc sources for the conventional MLI. The relative values between the upper (auxiliary) inverter dc voltage and the lower H-bridge (main) inverter is 1/3 resulting in 9 levels MLI. Each isolated dc source comes from a PV module accompany with a BESS. A high frequency transformer (HFT) is proposed in this paper to generate the dc bus voltage of the auxiliary inverter from the main inverter dc bus voltage. As a result of that the number of isolated dc bus voltages required is reduced by half which is reflected in reducing the number of PV required and thus reducing the cost of the overall system. In addition, a natural balancing between the main and auxiliary inverters dc bus voltages of each arm will be attained due to the HFT turns ratio which result in simplifying the overall control of the whole system. In order to reduce the size of the HFT a judicious modulation index (MI) will be chosen. The proposed system has been simulated using static inductive load. Analyses and simulation have been proposed to validate the proposed control scheme.